1. Field of the Invention
The present invention relates to a method of manufacturing a glass preform.
2. Description of Related Art
In the related art, in the case of manufacturing a glass preform by dehydrating and sintering porous silica glass, a chlorine-based gas such as chlorine (Cl2) or thionyl chloride (SOCl2) is used as a dehydrating agent of porous silica glass. In Published Japanese Translation No. 2004-523454 of the PCT International Publication and Japanese Patent No. 4540034, a method of sintering porous silica glass under a reducing atmosphere containing carbon monoxide (CO) without chlorine with respect to a conventional method of using chlorine gas is disclosed.
In addition, optical fibers manufactured using silica glass in which alkali metal oxide or alkaline earth metal oxide is doped has been shown by a number of predecessors to reduce the transmission loss thereof (for example, M.E. Lines, “A possible non-halide route to ultralow loss glasses”, Journal of Non-Crystalline Solids, 1988, vol. 103, pp. 279-288). However, a technique for industrially mass-producing such optical fibers is incomplete. In order to use a hydrolysis or a thermal oxidation caused by oxygen in gaseous phase in a conventional method for manufacturing an optical fiber, it is necessary to use, for example, gaseous raw materials, such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4).
However, alkali metal ions or an alkaline earth metal ion which are so-called hard cations form very strong ionic bonds. For this reason, most of compounds (salts) formed by such bonds becomes solid at around ambient temperature and ambient pressure. Accordingly, since the salts which become gas are hardly formed, it is difficult to apply the salts to the manufacture of optical fibers. Therefore, in order to commercially produce optical fibers in which alkali metal oxide or alkaline earth metal oxide is doped, a manufacturing method different from the methods conventionally established in this field should be developed.
Various attempts have been taken in regard to such problems. For example, a method utilizing the characteristic that an alkali metal compound or an alkaline earth metal compound is easily soluble in water, mixing an aqueous solution of the alkali metal compound in the form of an aqueous mist in a source gas to introduce the mist into an oxyhydrogen flame, and performing hydrolysis simultaneously with other raw materials to form glass has been attempted. In addition, it is known that composite salts obtained such that a certain kind of alkali metal compound reacts with other metal compounds have higher vapor pressure than the original alkali metal compound, and a method using the composite salt as a raw material has been attempted.
In further recent years, a method of strongly heating an alkaline metal halide to generate alkali metal vapor, exposing an optical fiber precursor glass to this alkali metal vapor, and doping alkali metal to the glass has been attempted. Moreover, a method in which an alkali metal halide is heated and vaporized, and then immediately cooled to form small particles so that a raw material is provided as an aerosol by transporting the particles using an appropriate gas flow has been attempted.
In the related art, chlorine gas or thionyl chloride used as a dehydrating agent of porous silica glass is respectively a gas or a fuming liquid, and has high toxicity to the human body. Therefore, it is necessary to take adequate measures to prevent a leakage from an apparatus and it is difficult to handle the chlorine gas or thionyl chloride.
In addition, regarding a method of doping silica glass with an alkali metal oxide or an alkali earth metal oxide, the above-described method in which the aqueous solution of alkali metal compound is used is a contradictory manufacturing method from the viewpoint that when manufacturing the optical fiber, usually, water should not be introduced into the glass. If water is introduced into the glass, a transmission loss of the optical fiber increases. Moreover, in the above-described method in which the composite salts having higher vapor pressure is formed and vaporized to be introduced, the vapor pressure increases minutely and thus, the desired effect cannot be obtained. In addition, since chemical species which are unnecessary for performing a desired function of the optical fiber, there is concern about increased transmission loss.
Furthermore, the method of strongly heating an alkali metal compound and obtaining alkali metal vapor has an indefinite reaction mechanism of a reduction, and is deficient in practice. In the above-described method of using the aerosol, it is not easy to control the amount of raw material supply in comparison with the method of the related art in which gas is used as a raw material.